1. Field of Invention
The field of the invention relates to a bath for electroplating nickel-phosphorus alloys coatings to conductive substrates and processes utilizing such baths.
2. Description of the Prior Art
Nickel phosphorus (NiP) alloys are coatings which exhibit good corrosion protection and excellent wear properties. The automotive industry applications include use on decorative parts such as exposed trim where both corrosion protection and estestics are important. Engineering applications include those where wear protection, corrosion protection and/or estestics are important, for example, coating of internal tanks and vessels such as fuel tanks, piston rings and shock absorbers. Many tools are coated with a NiP alloy to increase their service life because of the hardness of the NiP coating compared to pure nickel coatings. Additional uses are found in catalysis and electrical applications. There are several methods to produce NiP alloy coatings. One is rapid quenching from a melt containing nickel and phosphorus. Another is vapor deposition which produces NiP alloys containing variable amount of occluded phosphorus.
Perhaps the most widely used method to produce NiP alloys is through electroless deposition which uses a hypophosphite anion as a reducing agent in the plating solution. During the deposition of nickel on a conductive substrate, the hypophosphite may be reduced to elemental phosphorus and is occluded in the nickel coating. However, the plating rates from an electroless nickel solution are very slow compared to electrolytic deposition. Therefore, several studies have investigated the use of DC plating to deposit NiP alloys at higher deposition rates. It has been found that nickel deposits with up to 18% phosphorus in the coatings are possible through variations in the phosphorus containing species and current densities.
The electrolytic solutions in the literature employ nickel sulfate as the source of nickel. A typical nickel sulfate solution contains nickel sulfate (about 150-250 g/l), nickel chloride (about 45-50 g/l), phosphorus acid (about 10-50 g/l), phosphoric acid (about 0-40 g/l) and boric acid (about 0-10 g/l). The operating pH is between about 0.8-10 and the temperature may vary from 50-75.degree. C. In order to facilitate and maintain a high consistent deposit a high temperature and usually high agitation of the electrolyte is necessary.
It would be desirable to have baths other than nickel sulfate from which to electrodeposit nickel for several reasons.
One is that the amount of the phosphorus acid or phosphoric acid used in the prior art electrolytic baths is critical. If there is too high a concentration, the deposition of nickel is inhibited. If the concentration of either phosphorus acid or phosphoric acid gets out of balance, the only means to bring them back in the normal range is to dilute the bath. It would be advantageous if the concentration of phosphorus acid and phosphoric acid could be adjusted without diluting the bath.
Another is that since energy costs are an important consideration in the electroplating industry it would be advantageous to use an electrolyte which had higher conductivity than the baths of the prior art, thereby requiring less voltage for the same current density. Higher conductivity of the bath is especially important in high speed plating using a narrow anode-cathode spacing.
Accordingly, the present invention seeks to obtain the advantages of avoiding these and other difficulties encountered in the related art. These and other advantages are obtained according to the present invention which is the provision of a process and composition of matter that substantially obviates one or more of the limitations and disadvantages of the described prior processes and compositions of matter of the related art.